A Non-PRE Double-peaked Burst from 4U 1636–536: Evidence of Burning Front Propagation

We analyze Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array (PCA) data of a double-peaked burst from the low-mass X-ray binary (LMXB) 4U 1636-536 that shows no evidence of photospheric radius expansion (PRE). We find that the X-ray-emitting area on the star increases with time as the burst progresses, even though the photosphere does not expand. We argue that this is a strong indication of thermonuclear flame spreading on the stellar surface during such bursts. We propose a model for such double-peaked bursts, based on thermonuclear flame spreading, that can qualitatively explain their essential features as well as the rarity of these bursts.